Two wild grasses, perennial teosinte, Zea diploperennis Iltis, Doebley and Guzman, and Eastern gamagrass, Tripsacum dactyloides L. have been crossed to produce a fully fertile bridge species that may improve corn, Zea mays L., by conferring beneficial traits such as pest resistance and drought tolerance. Z. diploperennis (hereafter referred to as diploperennis), was an unkown wild relative of corn until it was discovered, apparently on the threshold of extinction, in the mountains of Jalisco, Mexico in the late 1970's. It is in the same genus as corn, has the same chromosome number as corn (n=10), and hybridizes easily with it. Gamagrass is a more distant relative of corn with a different haploid chromosome number (n=18), and varying ploidy levels ranging from 2n=36 to 2n=108. Tripsacum has been crossed with corn by artificial techniques and hybrids obtained are male sterile and essentially female sterile. All attempts to cross Tripsacum and annual teosinte, the closest relative of corn that some scientists believe is its wild progenitor, failed. Many plant breeders believe that Tripsacum has significant potential for improving corn by expanding its genetic diversity.
In 1984, crosses were made by pollinating diploperennis with pollen from a tetraploid (2n=72) T. dactyloides. U.S. Plant Pat. No. 6,906 for Sun Dance, the hybrid from that cross, was issued Jul. 4, 1989. In April, 1985, the reciprocal cross to the tetraploid Tripsacum was made using diploperennis pollen, and U.S. Plant Pat. No. 7,977 for Tripsacorn, the hybrid from that cross, was issued Sep. 15, 1992. U.S. Pat. No. 5,330,547, a utility patent on the method for transferring Tripsacum nuclear and cytoplasmic genes into maize via Tripsacorn, was issued Jul. 19, 1994. Ser. No. 08/248,333, filed May 24, 1994, is a continuation of the utility patent involving hybrid material, derived from a Tripsacum female parent pollinated by diploperennis, in crosses to maize.
On Jan. 18, 1988, pistillate inflourescences on a diploperennis plant were pollinated with pollen from a distinctly different T. dactyloides that is a diploid (2n-36) rather than a tetraploid plant. The seed was harvested and stored until Jan. 18, 1992, when it was germinated. The seed had been stored for four years because it was pale in color and did not appear viable. Earlier attempts to germinate seed obtained from crossing diploperennis and Tripsacum plants that were not tetraploid had failed. The reason I attempted to germinate the seed in 1992 was I decided to cull all old seed from earlier crosses. As a precaution, I never throw out seed until I run it through a standard germination test. Much to my surprise a single seed from this cross germinated and grew to produce a normal, fully fertile plant that is perennial and produces viable fruits twice annually.
Sun Star has been propagated by rhizome divisions and cuttings. Crosses have been made to inbred corn line W64A. Sun Star is similar to Sun Dance and Tripsacorn, hybrids derived from crossing tetraploid Tripsacum with diploperennis, in that it is fertile and cross-fertile with corn. It provides another novel genetic bridge for moving genes from a different Tripsacum into corn, thereby establishing a link between these wild grasses and modern corn that may be beneficial in corn improvement breeding programs.
Sun Star is distinctly different from Sun Dance and Tripsacorn in that its Tripsacum parent is a diploid rather than a tetraploid and comes from a different geographical area than the tetraploid Tripsacum plants. Therefore, previously patented plants are not intimately related to the invention of this material which used a parent plant with different qualifications. Unique propagation of Sun Star through successive generations by means of cuttings has demonstrated that the new plant has not only retained the continuous and abundant production capability, but also that its distinguishing characteristics hold true from generation to generation and appear to be firmly fixed. Propagation has taken place in Durham, N.C.
In bioassays, Sun Star shows distinct resistance to the larvae of Western corn rootworm, Diabrotica virgifera, as indicated by no root feeding damage and no larvae retrieved. In a field test during August, 1995, after the plant went without water for a week with temperatures exceeding 90.degree. F. during the day, Sun Star was still green and appeared as though it had been watered; whereas, under the same conditions, Tripsacorn, Sun Dance and diploperennis suffered severe wilting and most of the vegetative growth turned brown and died. In addition to corn improvement, other obvious utility for Sun Star includes potential as a perennial forage crop on marginal land, use as a ground cover, and prolific production of novel grain.